1. Field of the Invention
The present invention relates to a device for changing the copying magnification in a copying machine which is capable of scaling up and down the size of a copy as required.
2. Description of the Prior Art
Nowadays, many copying machines, either the manuscript fixing type or manuscript moving type, incorporate magnification changing devices. A typical known magnification changing device has a combination of a constant-speed mirror and a half-speed mirror which are movable at a speed ratio of 2:1 to change the positional relationship therebetween, and a lens which moves in proportion to the change of the positional relationship between these mirrors. In another known magnification changing device, a plurality of lenses mounted in the body of the copying machine are used selectively to provide various degrees of magnification.
Anyway, in the magnification changing device, it is necessary that the distance a between an object and a magnification lens and the distance b between an image and the same lens are varied to meet the following condition in order to obtain the desired degree of magnification which is given as the ratio a/b, i.e. the ratio of the distance a to the distance b: EQU 1/a+1/b=1/f
where, f represents the focal distance of the lens. In order to move the mirrors and lens while satisfying the above-mentioned condition, the device incorporates independent cams or similar actuating means as well as power sources such as motors, for the respective mirrors and lens. Consequently, the mechanism of the magnification changing device is highly complicated thus raising the cost of the device.
In addition, the following problem is encountered when the magnification changing device employs a cam mechanism for setting the positions of the reflecting mirrors and the lens for the purpose of attaining the desired degree of magnification. Namely, in such a case, the design of the cam becomes difficult particularly when the magnification degree is around 1.0. Thus, the known magnification changing device incorporating the cam mechanism is capable of effecting only a limited amount of change in the degree of magnification thus limiting the range for scaling up and down the copy size.
Furthermore, the known magnification changing device generally has a large size which increases the size of the copying machine as a whole.
It is to be pointed out also that the changing of magnification is possible only by fixed amounts, requiring troublesome manipulation when moving the mirrors and lens to the preselected positions corresponding to the desired magnification degree. Namely, the known magnification changing device is not constructed such as to permit focussing by means of a freely selected change in the degree of magnification. Thus, the known magnification changing device is still unsatisfactory from the user's point of view.